An occupant detection system for detecting an occupant on a seat of a vehicle includes an antenna electrode in a seat bottom of the seat, and an electronic unit connected to the antenna electrode and the vehicle. The electronic unit applies a load current to the antenna electrode so as to generate a weak electric field, and detects a potential current passing through the antenna electrode. The electronic unit detects an occupant based on an impedance and a phase difference, which are based on the load current and the potential current.